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10-14-2012, 10:55 AM   #1
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K-5 reporting wrong aperture for A setting

Hi,

I recently got a new lens with A setting. Which worked fine for about a week, but then this morning when I am in the A setting (it is an otherwise manual lens), the aperture is reported 1 stop off. The lens goes from 2.8 to 22, but the reported aperture only goes down to 5.6, and up to 32.

No amount of cajoling (lens out/in, change lens, power off, battery out, wipe contacts, etc. helped. The iris seems to be moving as it should and snappy.
I tried 2 other manual A lenses, and two new AF lenses and they all seem to report their apertures correctly.

Now, this has happened once before with a different lens, and it got fixed when I removed and remounted the lens (it was a f/2 being reported as f/4 minimum). That lens is working fine now, so I am hoping it is the lens and not the body.

I don't know how the body knows what the aperture range is on old lenses, so don't know what to try to fix, clean or otherwise inspect.

Any ideas?

10-14-2012, 11:22 AM   #2
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Which lens is it exactly?
10-14-2012, 11:26 AM   #3
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It is a Sigma 24 f/2.8 Super-Wide II
10-14-2012, 11:36 AM - 1 Like   #4
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Looking at this chart, it seems that contacts 2 and 3 are insulators while they should be conductors:
http://www.robertstech.com/matrix.htm

Either they are dirty or there is something wrong inside the mount of the lens. Check for continuity between contacts 4 and contacts 2 and 3 if you have a multimeter handy.

10-14-2012, 11:43 AM   #5
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QuoteOriginally posted by atekant Quote
It is a Sigma 24 f/2.8 Super-Wide II
My super wide II decides that it is an f4 in hot weather. Cool it down it's 2.8 again. It also had a problem one-time where the aperture lever became internally bent which made it so it would never open completely -- I was able to take off the mount and fix that, I'm pretty sure that was caused by temperature change also. Some material in there seems to be expanding/contracting with heat/cold more than normal. I have not tried to fix the changing aperture readout thing -- just laziness and now I'm probably good until Spring...
10-14-2012, 11:56 AM   #6
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My Sigma 20/1.8 tells my K-5 that it goes down to F1.7, but I know that the lens is a chronic liar.
10-14-2012, 12:02 PM   #7
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Wow! That is an excellent resource that explains how the A setting works. Thank you for that.

As for the lens, the contacts are definitely not dirty - not any more.
I don't have a multimeter, but will borrow one (or use this as an excuse to buy one) and check the contacts.
If it is something inside the lens, I will probably let it be, and use it in M mode instead of trying to open it up.
10-14-2012, 12:09 PM   #8
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QuoteOriginally posted by vonBaloney Quote
My super wide II decides that it is an f4 in hot weather. Cool it down it's 2.8 again.
Huh! That might explain it, as just yesterday, the weather turned, and we went from high 70s to mid 50s. Maybe mine behaves just the opposite to yours and will report f/5.6 in cold (I also saw a 4 when fiddling with it this morning). Except it is still doing it at home where it is warm.

10-17-2012, 12:29 PM   #9
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Hi, I had a similar problem recently with a lens, see:- https://www.pentaxforums.com/forums/general-technical-troubleshooting/197372-...acro-lens.html
You could maybe try some metal tape over the suspect contacts on the lens, as indicated by 'Elliott' above. It may be a longshot but might narrow down the issue before stripping it down.

Good luck....
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